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What happens to the pressure differential of airflow under and over the wing as the critical angle of attack is reached?
I've been doing a research project for mathematics which involves calculating the relationship between lift and angle of attack. While this may seem pretty easy, it's not for a 12th-grade student like me.
We all know that the pressure differential is 1/2 ρ〖(v2-v1)〗^2, where v2 is the airflow velocity over the wing and v1 the airflow velocity under the wing.
It is also fairly commonly known that as AoA is increased, so does (v2-v1) leading to a higher lift coefficient.
But, my question is: Whenever the critical AoA (AoA at which the aircraft stalls) is reached, how do we calculate pressure differential? What is the new figure for (v2-v1)? How do these airflow speeds change as the critical AoA is reached?

After critical angle, the flow is not laminar any more, so Bernoulli’s principle doesn't apply.

@mins, the flow is never laminar at the Reynolds numbers involved in full-scale aircraft.

@mins, actually, when the flow is turbulent, the vorticity increases the velocity, which in perfect accord with Bernoulli's principle decreases pressure and increases the lift. That is why vortex generators work. Of course it complicates the analysis.

@mins, the thing that happens in stall is that the flow turns backwards above the wing. This may cause a row of vortices to be shed, but it can also be stable. You can see similar effect in a river—near the bank the water sometimes flows backwards and especially in shallow bays such flow is steady.

While this may seem pretty easy, it's not for a 12th-grade student like me.

It definitely does not seem easy. It takes solid background in numeric integration, good working knowledge of a numeric library and some serious programming time.
There is a good reason everybody—and I mean aircraft engineers and researchers—just uses XFoil. And that's just the basic analysis—3D calculations require even more complex software packages—that cost hefty sums of money (XFoil is free).

We all know that the pressure differential is 1/2 ρ〖(v2-v1)〗^2, where v2 is the airflow velocity over the wing and v1 the airflow velocity under the wing.

There is no single $v_1$ and single $v_2$. There is just $\vec{v}(x, y)$ (limiting to 2D analysis like XFoil does for simplicity) that is different at each point. You can't just take $\alpha$ and divine two speeds from it, because the situation is more complex than that.
Remember that:

The air flowing over the wing reaches the trailing edge in significantly shorter time than the air flowing below it!
A thin flat plate is not a particularly good wing, but it does generate some lift.
At the Reynolds numbers involved for full-scale aircraft the flow is rarely laminar beyond 20–30% chord. Turbulence increases the velocity and therefore decreases pressure and increases lift.
In viscous fluid, the velocity must be continuous in all coordinates and that includes the boundary, so the layer directly touching the surface is not moving relative to it. The velocity quickly increases in the boundary layer. The properties of this boundary layer is what governs whether the stream will remain attached or not.
There is no lift in inviscid fluid (= superfluid) and neither would there be in massless fluid. That is, you need to take both inertia and viscosity into account to get any result.

Basically the only way to calculate this is by evaluating the Navier–Stokes equations, which has to be done numerically and in fairly fine grid to achieve any useful precision.
Before numeric integration, there were some simpler analytical methods like the thin airfoil theory, but those didn't work from scratch—some coefficients must be measured experimentally.

Whenever the critical AoA (AoA at which the aircraft stalls) is reached, how do we calculate pressure differential?

Just like before—by integrating the Navier–Stokes equations.


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Why are the boundary layer suction holes placed far forward from the engine?
Edit: I know that the suction holes reduce the boundary layer; I am wondering if there is a reason why they are placed far ahead of the engine’s intake. My thinking is that the long duct past the holes will form its’ own boundary layer, hence reducing the effectiveness of the holes.
Looking at the picture of the Eurofighter inlet, the suction holes for the boundary layer are located far forward from where the engine actually starts.

Source
Another example is the SR-71 intake - the suction holes are located in the middle of the spike which is also located far ahead from the engine:

Source
Why are the suction holes not located closer to the engine?
Surely, the long duct aft of the suction holes will develop its' own boundary layer - which is what the suction holes were reducing in the first place?

@ymb1 - I disagree. Your linked question explains what the holes are for and their purpose. This question indicates knowledge of the "what are they for" and instead asks why they're placed where they are in the intake, as opposed to other locations (farther back from the intake lip) within the intake.

Possible duplicate of What is the function of the holes in splitter plates?

I understand their function. I am asking if there is a reason why they are not placed closer to the engine. I edited my post accordingly.

Ideally, the engine inlet supplies the engine with appropriate air-flow across the operating range of the aircraft while incurring a minimal loss in total pressure. Under supersonic operation, shocks form near the inlet lip. The shock wave boundary layer interaction (SWBLI) thickens the boundary layer around the shock, develops an adverse pressure gradient, and increases total pressure loss. Boundary layer suction is used to prevent further pressure losses from flow separation due to the adverse pressure gradient. So, suction removes low-energy air to help control SWBLI near the lip and subsequently improve inlet pressure recovery.
From (Titchener 2013) specifically relevant to the SR-71:

In inlets where there are multiple SWBLIs distributed suction is often employed. Fukuda et al. (1977) suggest that a major reason for this lies in the complexity of these inlets which leads to a heavily iterative bleed system design. In such a case, a model with a large number of distributed holes that can be easily switched on and off helps to lead the design to optimal solution more quickly. Consequently, the complex mixed compression inlet system on the SR-71 (see Smeltzer et al. (1975)) and the Boeing/NASA SST inlet (see Tjonneland (1971)) both utilize substantial areas of distributed suction. As do the F-4, F-14, and F-15 which have multiple compression ramps on which distributed suction was deemed necessary.

The following are good references for further reading (included the thesis since the first link requires access to AIAA papers):

R.K. Scharnhorst, An Overview of Military Aircraft Supersonic Inlet Aerodynamics, AIAA 2012-0013, 2012.
N. Titchener, An Experimental Investigation of Flow Control for Supersonic Inlets, Ph.D. thesis, University of Cambridge, 2013.

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How does the aircraft keeps propelling forward when airflow and thrust are not aligned?
Assume an aircraft is flying at an angle of attack of 10 degrees, and that the incidence angle between the aircraft fuselage and the wing as well as the angle between the wing and the engine mount is also 0 degrees, the thrust force is at 10 degrees relative to the incoming velocity. My question is, since there exists an angle between the incoming the airflow and the thrust force, how does the aircraft keeps propelling forward straight?
I mean if I break down the thrust force into its components, then one component can be added in the direction of lift, and other component can be used to counteract the drag. But why would this make the aircraft to keep moving in a straight path? Because aircraft itself doesn't know how the component breakdown should be established, and also it just sees a resultant thrust force. Moreover, theoretically, I can make the component breakdown of the Thrust force using a different co-ordinate system, but still why would the aircraft keep on propelling in the same straight direction?

In such a case, Lift is less than Weight.  See https://aviation.stackexchange.com/questions/40921/does-lift-equal-weight-in-a-climb/56476#56476 , second-to-last paragraph, for an actual equation, not limited to horizontal flight by the way.  Surely this question may be a duplicate of some other?

Let me check out the question you attached. If it is duplicate, what should I do?

The plane moves straight because the pilot is telling it to do so.

@RameezUlHaq To arrive at a steady flight state, the aircraft initially needs to accelerate to this stage, so until it is reached, forces are not in equilibrium. Once it has attained the desired state, forces are reset by use of throttle and stick so they are in equilibrium. To move on in a straight path does not require more forces than those which balance out.

You don't break down forces, you sum them up, divide by mass and get the acceleration. And then you sum their moments, divide by moment of inertia and get the angular acceleration. Whenever both sums are zero (because pilot set the controls to make them so), the aircraft flies straight. Breaking down forces is only a mathematical tool to make summing them up easier.

Let us assume level flight. Then the forces acting on the aircraft are shown in the following sketch (not necessarily to scale):

The forces are :

the total aerodynamic force $ F_A $, which is split into two components: lift $L$ (perpendicular to direction of motion) and drag $D$ (parallel to direction of motion)
the weight $W$
the thrust $T$, here acting slightly upwards because of the angle of attack $\alpha$

For un-accelerated level flight, the sum of all forces must be zero. This is independent of the coordinate system you use to split the forces. In our example, this is given by these equations:
$$ L + T \sin \alpha = W $$
$$ T \cos \alpha = D $$
At a given angle of attack, these equations will only be true at one particular airspeed. At higher airspeed, the lift would increase resulting in a climb, which then changes the angle of attack. At lower airspeed, the lift would decrease resulting in a descent.

What I mean to say is this: Since, in a level flight, all the forces acting on the aircraft are zero. But why does the aircraft propels in the direction opposite to drag? It can move in any direction with constant speed. But why only in the direction opposite to drag?

A net force causes a change in momentum (and therefore at fixed mass, velocity), but it does not have anything to do with the current direction of the velocity. The aircraft keeps flying forwards because that is the direction it was flying in before. The net force equal to zero just means no change.

By the way: drag is by definition opposite to the direction of motion (relative wind).

So in order for the pilot to make an aircraft fly in a specific direction at constant speed, he needs to make use of some control surfaces to achieve that desired direction of motion, and as soon as it is achieved, he should immediately alter the control surfaces such that the overall forces on the aircraft again becomes zero? Subsequently, the aircraft will be able to move in that desired direction. Am I correct?

@RameezUlHaq Yes, that is exactly what the pilot does: use control surfaces to fly the aircraft and once you are on your desired path, you trim the aircraft to keep the net force to zero without control inputs.

The pilot controls the direction of flight with elevator and throttle.  Pitch to the horizon is determined by this combination provided the aircraft is making enough vertical lift to equal its weight.
So, to get level flight, one would reduce AOA and/or throttle until the sum of the vertical lift components equals weight at zero pitch (to the horizon) angle as shown by the lift equation:
$Lift$ = Area x Density x Coefficient (camber/AOA) x V$^2$
Just like any other plane, to climb, add power, to descend reduce power.
We must take note the drag vector in the direction of flight must equal the thrust vector contributing to velocity in a realistic force diagram.  The decomposition of the thrust vector is done to show the vertical component contributing to vertical lift.  "T-D" illustrates how much of the total thrust vector is available for the creation of airspeed.  Graphing Vy and Vx will show a greater value of "T-D" for a given amount of Thrust at Vy.
And here (finally),  a vector diagram, drawn to scale.  Note how much drag the angled wing lift vector is making.

So one may muse: why not leading edge SLATS for climbing.  Moving the Lift vector to vertical makes T-D = ... T!

"We must take note the drag vector must equal the thrust vector in a realistic force diagram." Almost correct: the drag vector equal the component of the thrust vector parallel to the drag vector (i.e. total thrust must be higher than drag if the engine was mounted at the wrong angle). Normally this little detail is insignificant but since you explicitly mention engine angle, I thought you may want to be slightly more precise.

"Pitch to the horizon"-- seems to mean pitch attitude but could edit to be more explicit. "provided the aircraft is making enough vertical lift to equal its weight."-- seems to imply aircraft will not reach equilibrium until vertical component of lift vector equals weight, which is not exactly true even in horizontal flight, if there is upthrust or downthrust. Maybe answer meant to say "provided the sum of upward vertical force is equal to weight"?

Re "We must take note the drag vector must equal the thrust vector in a realistic force diagram."-- not exactly, even in horizontal flight, if there is upthrust or downthrust.  Rather, for horizontal flight (and only for horizontal flight, we can say the horizontal component of thrust is equal to drag.

Re "Downthrust is added to most GA aircraft engine mounting, which avoids this clumsy configuration of forces and allows the aircraft to fly more efficiently on its wing."-- arguably, for whatever horizontal cruising airspeed the wing incidence angle (relative to fuselage) is optimized for, zero downthrust would make the most the most efficient use of the thrust vector, by keeping it purely horizontal.  The downthrust may be there for other reasons, relating to pitch stability characteristics, compensating for factors such as varying downwash over tail at various thrust settings, etc.

Could be a good ASE question-- "Presuming that the wing incidence angle is optimized to streamline the fuselage in some specific regime, why design any downthrust into the engine mount?"  A quick look over the site suggests that this would not be duplicate of any existing question.

@quiet flyer good points, I mean all vertical components equal weight, including thrust.  (I am seeing a vertical lift totem pole, with lift, thrust, drag, and...thermals!).  And yes, downthrust takes the edge off of excessive static stability when a blast of throttle is added.  Plugging numbers for the "triangle" has yielded good results compared with actual max climb rate for a 172 (around 750 feet/min).  Comparison of sin thrust vs cosine at 5 degrees shows a surprisingly strong vertical contribution with little loss of horizontal thrust component.

@quiet flyer but I am a still a little worried about the model because adding thrust and pitch are strong lifting inputs.  When you think of what is done leveling off into cruise from Vy (reduce AOA, reduce throttle, trim down) and the damn thing still picks up 30 knots in air speed. So, like many models, it's a good start, but I am wondering if a little vertical drag would cover a little excess lift (to close the vectors.  Thrust, drag, vertical lift, and weight (any sin or cosine contributors as well) should always cancel.

It appears that you've added a vector diagram that has the wing's lift vector not being purely vertical.  Why?

Re "but I am a still a little worried about the model because adding thrust and pitch are strong lifting inputs. When you think of what is done leveling off into cruise from Vy (reduce AOA, reduce throttle, trim down) and the damn thing still picks up 30 knots in air speed"  -- you've reduced the lift coefficient, so the airspeed must increase, or else the plane would be accelerating downward.

@quiet flyer 1.  Because the plane is flying at "an AOA of 10 degrees".  2. Because somewhere between up and down is level.  Time for a break.

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Is the total lift of a quadcopter the sum of the lift from each propeller?
In this diagram each rotor R1,R2,R3,R4 has an individual lift force of 490 N. All combined, does the total force on this aircraft equal to 1960 N?
The aircraft in question is similar to a quadcopter.

Are R2 and R3 vertically separated, or horizontally?

Yes.  490 + 490 + 490 + 490 = 1960.  For practical purposes, the thrusts from individual rotors do not interfere constructively or destructively:  they neither cancel each other out, nor do they help each other.  This case is pretty much the same as multiple propellers mounted on a fixed wing.
The situation becomes more complicated only if some rotors overlap, or are coaxial.


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Why does pitching moment coefficient decrease with an increase in the angle of attack?
Slope of Cm vs alpha graph is negative, why it is so?
The attached graph is from https://en.m.wikipedia.org/wiki/Pitching_moment , this graph is for cambered airfoil, as pitching moment is zero for symmetrical airfoil, as center of pressure remains constant for symmetrical airfoil.
My question is different from suggested question Should the pitching moment be up or down? as my question is talking about pitching moment coffecient which depend on pitching moment, dynamic pressure, and other parameter.
Cm= M/qSc
Where M is pitching moment, q is dynamic pressure, S is the wing area, c is the length of chord of airfoil. But I am not able to get it correctly how Cm decreases with increases in angel of coffecient. If we see pitching moment value then I think it should increases with increase in angle of attack as centre of pressure shifts away from center of gravity with increase in angle of attack.
 (source: wikipedia)

Where does you graph comes from? to what aircraft type does it apply?

That looks like the Cm of the whole aircraft. Is that right?

@Manu sir source of graph is https://en.m.wikipedia.org/wiki/Pitching_moment

@JZYL sir, I also think it's for whole aircraft.

Does this answer your question? Should the pitching moment be up or down?

@JZYL sir, I am new to aerodynamics concept, might be my question wouldn't be very clear so I edited it. Sir I think it's bit different from your suggest question. But even this question helped to learn more about pitching moment. Thanks

@GouravSingh I put a link to the original graph in the question as it contains context.

There are several, equivalent explanations:

The airplane has positive static longitudinal stability.
As the angle of attack increases, wing lift grows more slowly relative to tail lift because the coefficient of lift on the wing is larger than that on the tail.
The airplane exhibits a growing nose-down pitching moment as the angle of attack increases in order to return to its trimmed angle of attack of -3°.

The increasing slope of the moment curve at 12° angle of attack indicates stall onset on the inner wing. Here, the flow starts to separate on the rear upper wing root which reduces local circulation and, consequently, the downwash angle at the tail so the lift increase on the tail becomes disproportionally large. This is typical for a straight wing with a moderately high aspect ratio.
The two lines show the behavior at two different Reynolds numbers which seem to belong to a model airplane or a low-speed tunnel measurement. They also show the behavior of a full airplane, not an isolated airfoil.

If we see pitching moment value then I think it should increases with increase in angle of attack as centre of pressure shifts away from center of gravity with increase in angle of attack.

It does grow, but in the negative direction. This is because a nose-up pitching moment is defined as positive and what we see here is an increasing nose-down pitching moment. The center of pressure of the wing-tail combination is shifting backwards with increasing angle of attack and the pilot would need to trim a negative elevator deflection to bring it back to the center of gravity.

My question is different from suggested question Should the pitching moment be up or down? as my question is talking about pitching moment coffecient which depend on pitching moment, dynamic pressure, and other parameter.

No, it's not. As the constant Reynolds number indicates, this has been measured at constant speed and it does not matter if it is a coefficient or the "real" moment because both only differ by a constant factor.


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When does an UAV need an internal structure?
I´ve seen some UAV models without internal structure (stringers, longerons and frame). I guess the drone´s shell is the frame itself. So what is the best approach to calculate the shell thickness to avoid using a more complex internal structure?
Data:

Wingspan: 2.4m
Length: 0.9m
AR: 11
Speed: 16m/s
Estimated weight: 3.2 kg
Fuselage and wings materials: carbon fiber

Is the shell made out of steel, or paper mache?

Carbon/ glass fiber

Welcome! You've taken the tour, now browse the [help] to see how to best ask a question to get a good answer. "What's the best approach" questions are very broad and open ended and tend to receive very opinionated answers and are usually closed because of that. If you can provide some details of the design you're working on with a specific, answerable question, someone (or 6) will comment on it. Also, please [edit] the additional info into the question. Not everyone reads all the comments and comments can get deleted at any time.

The terms you are lacking are "monocoque" and "semi-monocoque", two different ways to build a structure. The UAV you describe that didn't have visible longerons, etc was a either a monocoque structure, or what you were seeing was just a fairing and the load bearing elements were elsewhere. We cannot answer your shell thickness question without a lot more data, but maybe you meant to ask: "what is the upper practical MTOW limit of monocoque airplanes designs?" or something similar. In that case please reword it a bit to make this clear.

If you don’t get a good answer here, you might also try asking on the new drones.SE site.

I very much doubt that you will hit your mass target (kg is a unit of mass, btw) if you build those wings without a proper spar. The shell is for carrying shear loads to provide torsional stiffness, but it cannot give much bending stiffness, especially if there is no internal structure that keeps the upper shell apart from the lower one under bending loads.
So build a light shell and add spar caps, a web and a couple of ribs where loads are locally introduced or where cutouts (say, for ailerons) start.
Regarding the fuselage: If there are many point loads inside from the payload and systems (camera gimbal, engine mounts) it makes sense to have local reinforcements. Otherwise, a shell should be fine given the higher curvature of a typical fuselage. On the other hand, everywhere you have large radii or flat panels, a local stiffener will be advisable.
As for calculating the shell thickness: At that size I expect that handling loads and inertial loads from point masses will determine the minimum size. There is no simple formula and you need to balance ease of operation with performance. Depending on your priorities, the shell will become thicker for a more rugged device or thinner for a lighter but more delicate one.


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What process do flight attendants and pilots follow to bid for routes?
What process do flight attendants and pilots follow to bid for routes and how does seniority play into this?
Is it the same across multiple airlines/does IATA have a preordained process that airlines are required to follow?

Typically, lines of flying are published, with a minimum "bid line guarantee" for the month.  The company may have a minimum guarantee of 60 hours, for example, which is what's used for pay.  The bid line might be 78 hours, meaning a pilot awarded that line would get 78 hours of pay, minimum.  Pilots bid lines based on their own needs, pay, schedule, etc, and the bids are awarded by seniority.  Upper seniority may get their first choice.  Lower seniority may end up getting reserve lines or assigned to flying schedules that no one else wants.


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How to choose the right ILS when the runway has more than one?
I'm wondering how is the right choose when an runway has ILS x, Y, Z. What should I consider in order to pick one of those for landing?

Do you have example of such runway?

@ManuH KTUP for example has ILS-Y and ILS-Z. See e.g. here.

Welcome to [aviation.se]! Please take the [tour] and read through the [help] so you can learn how to make the most of the site. You've made a "thanks" comment on both of the current answers, I certainly hope you've also clicked the "up vote" arrow for them, too, as that's the "official" way of saying "thanks" (also, "thanks" comments are frowned upon). Please make sure you click the check mark next to the answer that helped you the most (especially as both appear to be quite helpful). This lets others know that a satisfactory answer has been given to this question.

When an airport has multiple ILS approaches to the same runway (usually labelled ILS-Z, ILS-Y, ILS-X, ...), they all end up using the same ILS for that runway, but they differ in how you fly the approach until final.
Consider e.g. KTUP, which has an ILS-Y and ILS-Z approach to runway 36. Here is the chart for the ILS-Y:

(source)
This ILS approach starts at an RNAV waypoint GAGYE and therefore lists "RNAV 1-GSP required" as a requirement. The ILS-Z approach looks completely different:

(source)
This ILS approach has two IAFs: CEVSI on the OTB 12-DME arc and the VERON NDB. It therefore lists "ADF or DME required" as a requirement.
So, which one should you choose?

During flight planning, you should plan to use the approach that best fits your aircraft's equipment. If your aircraft does not have ADF or DME installed, ILS-Z in the example above would not be possible. On the other hand, if your aircraft is not RNAV approved, you cannot fly the ILS-Y. If you can do both, then file a route that ends up at whatever IAF works best for you.
During the flight, it should be clear which approach you should fly based on which waypoint you end up at. In the example above, if had filed a route to the VERON NDB, then you would fly the ILS-Z. If you filed a route to a beginning of a STAR, then you fly the matching approach from the last waypoint of the STAR (KTUP in my example does not have STARs, but other airports with multiple ILS approaches might). This would usually be the RNAV variant, since most STARs are RNAV anyway.
If still multiple approaches are possible, ATC should clear you for one in particular, e.g. via "Cessna 12345, cleared ILS Yankee approach runway 36".

Thanks a lot for the answer! Really clear!!

Also, note that the ILS-Y in that example has lower S-LOC minima, which may mean the difference between getting in and not when the GS is OTS. In general, the Z will have the worst minima but also the lowest (or at least oldest) equipment requirements.

ATC will assign the correct approach.
There are many different reasons some airports have multiple different ILS procedures for the same runway:

Some have separate procedured depending on whether ATC will provide vectors to final approach or the pilot needs to fly a procedure turn
Some have different procedures because of terrain/obstacle clearance - different aircraft with different performance may need to follow different routes to stay clear of terrain
Some have different missed approach procedures depending on the traffic flow in the terminal area
Some have almost identical procedures, where the only different is which DME-station is used for reference
Some even have two separate glideslopes with different angles/aiming points due to noise abatement (Frankfurt is an example)

Some procedures are limited to certain times of day, aircraft type or operator approval - in which case it is probably pretty obvious which one you should fly. In other cases (for example identical procedures with different DME-stations) which procedure to fly will depend on the actual status of ground equipment.
In any case, ATC will tell you which procedure to fly. When a runway has more than one ILS procedure (typically designated X, Y, Z) ATC will tell you to expect and clear you for that specific approach. For example: Expect ILS-zulu approach runway 27 and Cleared ILS-yankee approach runway 05

Thank for the answer!!


text:
What is the vertical speed limit when altitude mode is engaged?
This question applies to Airbus' autopilot but may not be restricted to just this manufacturer (there must be equivalent modes on ATR, Embraer, Dassault Falcons, etc.)
For the vertical plan, the autopilot offers different modes. Among those, there are V/S where it tries to keep the selected vertical speed, and Alt, where it tries to keep the selected altitude.
When in Alt mode, if the aircraft is not at the required altitude, then the autopilot commands a vertical manoeuvre to reach the desired altitude. My question is: during this manoeuvre, is the vertical speed limited and if is this limit configurable in flight by the pilots? (If not, I imagine it is limited to what can be achieved in open climb or open descent modes.)
EDIT: The question comes from the hypothetical situation where aircraft is flying towards a target altitude and the pilots want to limit the vertical speed for any reason (e.g. a better vertical separation with another traffic)

It would help if you described the scenario/s where the annunciation is Alt but the aircraft isn't  at the required altitude, for e.g., after a massive change during altimeter set/reset.

What you are describing is the managed climb mode of the Airbus, where the Flight Management Computer (FMC) determines the optimum climb speed and the autoflight system will then pitch to maintain this speed.
From the A320 FCOM (Autoflight - AP/FD Vertical Modes):

Climb Mode (CLB)
CLB mode guides the aircraft in a managed climb, at either a managed or a selected target speed, to an FCU selected altitude, taking into account altitude constraints at waypoints. The system also considers speed constraints if the target speed is managed. [...]
The pilot can arm the CLB mode during the takeoff, go around, climb and cruise phases and engage it during the climb and cruise phases.

The vertical speed is therefore determined by the difference of the current and the target airspeed. All protections like stall protection based on angle of attack still apply, which may limit the actual climb rate.
The same logic also applies when descending instead of climbing, where the aircraft will maintain a given descent speed by pitching at idle thrust instead of climb thrust.
Once you get close to the selected altitude, the ALT* mode will take over, which follows a vertical speed instead of pitching for speed:

Altitude Acquire Mode (ALT*)
ALT* (green) mode guides the aircraft to acquire the FCU selected altitude.
ALT* (magenta) guides the aircraft to acquire an altitude constraint provided by the Flight Management.
  Once the aircraft has reached the altitude, the altitude mode (ALT green or magenta) engages.
Engagement Conditions
The mode engages when the aircraft reaches the altitude capture zone defined by the aircraft vertical speed
  (among other parameters).
[...]
Guidance
The ALT* mode has internal V/S guidance that is a direct function of the difference between present altitude
  and altitude target. This V/S internal guidance has priority over the target speed. Therefore if a thrust reduction
  occurs in ALT* (Engine out), the vertical speed order will not be modified and a speed variation will occur. The ALT*
  mode does not have a mode reversion that results from speed variation.
[...]
ALT* and ALT CSTR* modes have internal protections that decreases the vertical speed when VLS or VMAX is reached (VLS or VMAX becomes the priority target).
[...]
The system switches automatically to ALT (altitude hold) when the altitude deviation becomes less than 20 feet.

The vertical speed is therefore only limited by the internal guidance logic, but will be overwritten when VLS or VMAX are reached providing the usual flight envelope protections.

As I understand, pilots cannot limit vertical speed when in ALT* or CLB modes.

No, nothing is mentioned about limiting V/S in these modes. I guess you would have to switch to V/S mode to limit it, but it will still go to ALT* when approaching the target altitude.

Same answer for the 737. If I hit ALT while in a climb or descent, the autopilot will smoothly return to the altitude we were passing when I did so, but the rates it uses to do this aren't specified for nor adjustable by the pilots. The automation does what it needs to do to get back to that altitude, but only it knows the exact parameters it will use to do so, including any constraints (limits) on them. Things will, unsurprisingly, look different depending on the initial climb/descent rate.

It should be noted that any altitude constraints on waypoints will have an affect on the climb/descent rate.

@Gerry Not for a climb (at least in the A320). In descent, the FMGC might switch to a geometric descent path between waypoints with altitude constraints. There is no geometric climb mode though. It will just climb at target IAS and then level off (if necessary).

The situation behind your question, "the aircraft is flying towards a target altitude and the pilots want to limit the vertical speed for any reason," would arise when an aircraft is climbing or descending at a high V/S:
Though TCAS algorithms have become far more sophisticated, there are cases where the vertical closing rate could cause an issue with legally cleared traffic not far above (climb case) or below (descent case) the target altitude. In such case, pilots were cautioned to limit their V/S approaching the target altitude.
Here's an extract from a Eurocontrol document – ACAS II Bulletin N°21 (February 2017):

Key learning points this issue:

All RAs must be followed. The traffic in visual contact may not be the threat that triggers the RA.
TCAS II will generate RAs in 1000-foot level-off encounters if aircraft approach their cleared levels with high vertical rates or due
to combined vertical rates.
To reduce the chance of unwanted RAs, reduce the vertical rate to 1500 ft/min. or less in the last 1000 feet before level-off.
Particularly, if another aircraft is at an adjacent flight level.
TCAS II will issue RAs against all threat aircraft with altitude reporting transponders.

(consult the latest documents and procedures for actual inflight use)
V/S mode could be used, but it maybe more expedient to just override the auto throttle clutch for a few moments during the capture phase of the level out at end of a climb. During the descent case most systems would already have declutched the auto throttle allowing the pilot 'manual' control, i.e the ability to push the throttles without changing the engaged modes and the auto throttles would automatically re-clutch upon altitude capture. Changing the vertical mode to VS shortly before level off does not fit well with the FMC based automation philosophy and these automated systems are getting more and more sophisticated to accommodate such eventualities.
As an example one some Boeings (e.g. 747-400 and 777) the rate of climb during go-around is limited to 2000fpm by limiting the thrust to the thrust required to meet the required gradient, even though the airplane may be capable of 3 or 4 times greater than this (note: this limit is easily overridden by another press of the TO/GA switches).


text:
On how many routes an aircraft is used on a typical day?
I wonder how an commercial aircraft used to transport passengers is used on a typical day. I imagine several scenarii, but I cannot figure out if they are right:

take off in the morning from the company's hub to an airport and then get back and begin again all day long, possibly with a changing the crew when turning at the company's hub
from the company's hub, begin the day with a round trip to an airport A, then performing a round trip to from the hub to an airport B, and continuing with other airport until the end of the day
a triangular shape: a one-way leg from the hub to an airport A, then another leg from the airport A to an airport B, and then another from B to the hub, and continuing until the end of the day. A derivative could be the same with more airports (hub --> A --> B --> C --> D --> hub)
a long multi-leg route in one direction and then back (eg. A --> B --> C --> D --> C --> B --> A)

I assume you mean commercial passenger transport?

The answer heavily depends on the airline and the kind of business they're in.
I suggest using Flight radar, where you can search by aircraft registration number, and see 7 days worth of flight history.
An example, EI-DCP, from Ryanair: http://www.flightradar24.com/reg/ei-dcp

@JanDvorak Yes, question edited accordingly

What you're actually talking about are called "city pairs," not "routes." The Route is the path flown, while a city pair is the start and end point.

Actually, a commercial airliner can do almost any set of legs, assuming those are technically possible (range/payload/laws like ETOPS...) But the main fact that make it possible is demand. If you run an airline, you won't make routes that don't have demand to make profit.
Aviation is a complex world. To make profit, you have to consider a massive load of parameters. You can't for example position complete set of crew in every city/airport hotel of your airline routes then decide to create fantasy flights "a la carte". Crews can't fly continuously aswell, so you can't put crew in a 737 and make them non-stop flights around the country an entire week.
One main thing you have to keep in mind : It's not one or several persons who decide "I want to build an airline with X aircraft on Y routes". It's one A-B route that decide how many passengers are available on, then, an airline has the opportunity to operate that route (and struggle to negociate all the authorizations)
Here are some hints about how airliners deals with their environment around the world :

What kind of flights does the airliner, and what airframes does it have in its fleet ?

An airliner can start with one single aircraft, says a DHC6 Twin Otter, and makes regular flights between two airports in the middle of the ocean. You can have 10, 15, maybe 20 round trip a day, some well after sunset.
An airliner can have half a dozen ATR or Dash 8, and a set of routes with the proper demand. You can have routes like hub-A-hub, hub-B-hub, when the demand is enough to make each flight profitable. Sometimes, you can't have enough passenger a day on each route, so the airliner makes hub-A-B-C-D-hub the day one, then hub-D-C-B-A-hub on day two, and cycle the sets...
An airliner with dozens of (the same) aircraft (737 or A32x) can either have the same round trip hub-X-hub destinations every day, several times a day, airlines like Air France, Southwest, Lufthansa, Air China... or the same fleet, but much more destinations operated two or three times a week : hub-A-B-hub, hub-C-D-E-hub, hub-F-G-H-nightstop-G-F-hub... like Angola Airlines (domestic and regional)
Long range flights (7 hours or more) requires the appropriate aircraft. If your airline can buy/lease such an airliner, the route hub-destination-hub can be operated in the limits of the demand. When the demand is enough to fill the plane along the week, then go ahead. Otherwise, the airline has to decide wether a) leave the plane on the tarmac, b) find a nearby complementary route : hub-A-B-hub, hub-B-A-hub, c) use a smaller and cheaper aircraft (if the crew is qualified on) d) combine destination A with a complementary destination B that fills schedules along the week (or bi-week) hub-A-hub-B-hub, e) replace the route with one better, f) negociate with one or several airliners to sell seats to them to fill the aircraft, g) negociate with one or several airliners operating on the destination airport to take advantage of the hubbing : make your airline land at the destination during rush hours.
Very long range flights can't be operated every day with one single aircraft. And very long range flights are a niche : the hours spent in that closed tube discourage passengers from taking such flights. And very long range flights are a hell to schedule : the aircraft does 3 or 4 flights a week, and stays on the tarmac the rest of the time.

The size of the airline :

If your airline is a big one, like British Airways or Japan Airlines or United, your reputation and historical operation is enough to get demand on most usual routes (and your staff has the expertise to select profitable ones) Most of your flights will be the type "hub-X-hub", either domestic, regional, or international.
If your airline is above average (like Kenya Airways, Aeromexico, Air India) you'll have a mix of "hub-X-hub" flights and some complementary legs to fix schedules and aircraft operation ratio : hub-A-B(-C)-Hub
If your airline is below average (Blue Panorama, Air Kazakstan, Belavia) chances are great to have charter flights, multiple flight shifts.. Timetables greatly vary from one year to another.
Small airlines like Air Austral, Republic Express (Indonesia), Insel Air will have historical regular flights, but most of them will be of type "hub-A-B-hub" and/or "hub-A-C-E-C-B-hub". Timetables also vary from one year to another, but at a less rate than above because the airline don't have enough aircraft to think of new or better routes to operate, as the ones available cannot be closed or replaced.

The region of operation - the airport statistics (passenger volume per year) :

There are regions of high frequency of flights : Western Europe, United States, Japan, South-East Asia, Eastern Australia, the Dubai-Europe axis... In those regions, you have several hundreds of round trip routes that can be operated ten times a day using a Boeing 747. Cheap flights, historical demand, dynamic business. You can operate flights of type "hub-destination-hub".
There are regions with much less traffic, but enought for one airline to operate the simpliest flights hub-dest-hub. However, once another airline or more decide to compete on the same route, everyone is forced to reconsider the way it operates their fleet.
You also have regions in the world, especially in Africa and some non-democratic countries out there, where only one airline is allowed to operate a given route. Usually, those airlines don't care about competition, as the state/country itself protect them. Ticket prices on those routes are unusually high, because they can. So the demand on those routes NEVER take off, thus, making the flights on them the same along the years. You'll have destinations operated once a week (if not once a month) You'll have very small airlines and you don't understand why, having the monopoly on the network, they are not able to grow, grow the traffic, and propose better schedules for custommers than the absurd routes and departure/arrival times. Simple : they simply don't care.

Night Flights - jet lag.

You can't just make an aircraft take off, and land when you want. A custommer usually don't want to land at an airport at 3AM.
An airline should avoid boarding passengers at 5:30AM, just because if you board at that time, you've waken up at 3:00AM, failed to take a breakfast, almost forgot your papers, arrives at the airport hungry and angry...
jet lag : If you take off from San Francisco at 09:00PM, going to New York, you'll land at 06:25AM. It's okay ! But if you take off at 05:15PM, you'll land at 03:40AM. You haven't slept well.. Perhaps at NY, it's okay to land so early in the morning, but try this in Charles de Gaulle and you'll see how boring (or expensive) it will be. Jet lag is an important factor for international flights. When the airline can't have the slots to land at a proper time at the destination airport, it should think of alternate flights to do, or negociate the opening of an intermediate destination of interest before operating the projected flight, this creating legs like "hub-alternal-hub-destination-hub" or "hub-alternate1-alternate2-destination-hub" or "hub-alternate-destination-alternate-hub". Airlines like Air Madagascar does this. Landing at an airport late in the night is another reason of nightstops. This is common with regional jets (737-ERJ-A32x...)

Aircraft operated by several airlines.

Sometimes, you'll have aircraft operated by the owner some days in a week, and by another operator the remaining time. Take for example InterAir of South Africa. The aircraft is based at Johannesburg, and operates InterAir flights from and to JNB. But on Wednesday and Friday morning, the aircraft takes off from JNB as an InterAir flight, to Cotonou, and from there, it operates for AeroBenin to several destination in central Africa. The aircraft returns from Cotonou the Thursday and Saturday afternoon as regular InterAir flight. So don't be surprised to find a flight from JNB to Cotonou Wed and Fri morning, and their returning legs on Thu and Sat afternoon. That's common in Africa, South America and Asia. Many airlines like Blue Panorama, Lot Polish, EgyptAir (...) lend some of their aircraft to smaller airlines in the world on a week basis like this. The smaller airlines operate them on routes like "hub-A-B-C-B-A-hub" to use the aircraft as long as possible in the few hours/days.

So, those are only hints to understand how routes as selected and arranged. To enumerate all possible schemes, even a book won't suffice.
It is possible to have much more aircraft than routes (when you have enough demand) or the inverse. An aircraft on the ground is an aircraft that costs money for nothing; so "hub-X-hub" flights are rarely the best way to operate an aircraft. Key words are "demand", "costs", "profit". To get factual datas to calculate those, you'll need parameters like aircraft performance, airframe availability, route statistics, crew/maintainance possibilities, security/safety, regulations/laws/authorizations... then compare and negociate the opening of the most profitable route with the less risks with the appropriate aircraft.
Side note : I'm no expert. I'm just an aviation enthusiast, mostly airliners of the 1960-today era.

Welcome to the Aviation SE! What an awesome first answer here. I look forward to reading other from you as well.

Thank you :) Just discovered Aviation SE a couple of days ago and found it very interresting. The other answers are also worth reading. There's a simulator/game called FlightSimulator or PreparED. You can import realistic flights from actual airliners. And there's a bunch of third parties addons/tools to edit, preview them, build stats, etc. I've my own made tool that shows flights on a map; that's how I visualize round trips, zig-zag shapped legs, bizarre hops, very short legs made by widebody jets. More common than I thought. Events like intl summits or tournaments also trigger unusual legs.

That'd depend highly on the aircraft and route network.
A 737 doing one hour hops might make 10-15 such in a day's job, a 747 flying a 14 hour leg won't make it back home the same day.
And then there's anything in between.
And whether that 737 flies the same legs over and over again or visits multiple destinations depends entirely on the owner's scheduling department, there's no hard and set rule (maybe it's a -900 which is needed between Amsterdam and London during morning rush traffic, then flies a longer flight to Tel Aviv in the afternoon, and in the evening hops to Berlin and then to Paris before being towed to a hangar for overnight cleaning and minor maintenance).

Depends, sometimes the schedule is tight so the aircraft do several trips without going to the hub. Some companies have designated aircrafts to one specific route (ex: A > B > A > B > A > B and so on). In my experience everything depends on company schedule, aircraft availability, crews avaliable, aircraft inspections, airport restrictions, etc

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What was the speed limitations for droop nose on the Concorde?
Lowering gears or extending flaps comes with limitations. Given the answer of this question I suppose the limit speed for full nose down is about 325 kt. My question is: what were the limit for all positions (visor down but nose full up, nose in take off position, nose in landing position, any other position I don't know)?


text:
What are L/D ratio of famous WWII fighters?
I'm looking for L/D ratio of old warbirds. I'm particularly interested by the P-51D, the Supermarine Spitfire Mk.VII and the Mitsubishi A6M3 Zero, but I'm also quite curious about other warbirds that flew during this period.
What are L/D ratio of those warbird? The answer may include L/D ratio of other WWII fighters.

Interestingly, resource location questions are usually off-topic, but this can be made on-topic by steering away from "where can I find..." and instead asking "what are the..." which will no doubt bag you some references.

@jamiec edited accordingly

Is a meaningful answer possible for aircraft that fly at a wide range of speeds and g-loads? For steady level flight, lift will equal weight and drag will equal thrust, but the drag/thrust will vary hugely between approach speed and max speed. Should we limit this to optimum glide angle, or some definition of 'cruise'?

Not a real answer, but you could try www.wwiiaircraftperformance.org it doesn't have the L/D ratios on the site itself but contains links to technical reports on the aircraft that may have that information


text:
What are the consequences of turboclair system on restriction due to turbulences?
A turboclair system basically consisted of jet engines used to warm the air around the runway and dissipate the fog. I understand that better visibility due to less fog allows less-well-equipped aircraft to land safely, but such an artificial temperature elevation may create strong turbulence (due to convection) and, thus, restrict the types of aircraft than can land safely (e.g., only heavier aircraft, which could be less affected by localised turbulence).
(I imagine that, nowadays, ILS is more effective than a fog removal system in bringing an aircraft safely to the runway.)
My question is: Because of probable turbulence due to this system, do operations into runways using a turboclair system come with additional restrictions compared to the same runway without fog?

We can look at the airport(s) in question, and see if the approach plates say anything about it. Do you have a specific one in mind?

Not according to this book.

This sounds terribly expensive.  Where has this been implemented? Anywhere?

@RyanMortensen according to this article it has been implemented at orly airport with 14 military jet engines.

Sounds like this could also be a way of generating some additional headwind to improve TO/landing performance. With a cost of added turbulence...

@pilothead why did you remove the keyword "turboclair" from the title? I also want the title to focus on operational procedure, not on hazard

I found a Turboclair addon checklist for the SE.2 Caravelle landing procedure and it says nothing about turbulence.  This procedure is intended for ferry flights without passengers and specifies the use of a braking parachute, so it is not for normal flights. It is possible that this was the checklist used during 1968-69 testing of the system before general approval and that a turbulence warning or restriction was added later, but it seems likely that any severe turbulence problem would have been recognized very early.
There are standard weather checks at the destination. There is evidently a Turboclair specific approach that must be requested and the pilot must announce arrival at outer marker, at 400ft, that he has landed and again when he has cleared the runway. He is to continue the use of instruments when on the ground due to poor visibility.

Initial testing in 1961 was conducted using modified De Haviland SE 252 jet aircraft as heaters, which could be re-positioned to find most effective locations.

Under construction from 1970 to 1974, the system's jet engines were permanently installed in underground pits. The reference shows the installation on Orly runway 07, remnants of which can still be seen in Google maps which now shows the runway as 06.

The system was also installed at Charles DeGaulle airport. It was used for fewer than 1000 landings between the two airports before being decommissioned in 1982.


text:
Can Flex thrust be lower than climb thrust?
Let's restrict this question to airliners with 2 jet engines.
For the remaining of the question, I'll use the term flex to designate reduced take off thrust as I'm more familiar with airbus's systems. Flex will be the thrust selected when thrust levers are in the flex detent and climb thrust the one selected when lever are on the "climb" detent without autothrottle engaged. I assume other manufacturers have an equivalent way to easily select those values.
Both flex and climb thrust are lower than TOGA thrust. In good condition (long runway, good weather, almost empty aircraft,...), how low can flex thrust be? Can it be lower than climb thrust?
In this case, what is the rational for accelerating from flex to climb thrust given flex is made to increase engine lifespan by reducing thrust?

Are you specifically asking about Airbus? FLEX thrust is an Airbus term, Boeing calls this Assumed Temperature Method.

@Bianfable I'm not asking for a specific aircraft. I should have speak of reduced take off thrust but flex was more convenient to write (this is a really bad excuse) (edited accordingly)

Post-edit (at least) this is a very clear and well written question.

Yes.  On the CF-34 family, in theory, a maxed-out flex setting can be in the mid 80s N1 whereas climb thrust is normally high 80s/low 90s, and you could see thrust rise when setting climb if an extreme flex setup (max Assumed Temperature) (or in the case of the non-fadec engines like the 3B1, the climb bug appears at a higher value than the flex take-off thrust bug when you select "climb" thrust in the FMS)
The reason is to do with what you might call the mission priority of the two settings, and where the actual benefit in flex lies.
With flex, the priority is distance and gradient for obstacle clearance.  The flex setting is the lowest setting that meets the takeoff and initial departure performance requirement for accelerate/stop and initial climb gradient for that runway and all up weight.  The benefit of using the lowest setting is only partly from lower temperatures; an even bigger factor is erosion from silica particles (outdoor dust) which is most severe below 1000 ft.  The dustier the environment, the larger the flex benefit. (In the Middle East, CRJ operators were wearing out engines after only 5-7000 hours because they were doing max thrust takeoffs all the time and had to be cajoled into using flex.)
Climb thrust has a time priority, from an operational balance-of-factors perspective.  You want to get to altitude in an optimized time-related profile (as soon as possible taking various things into account), and an optimized time-related profile can require more thrust than the minimum required to meet take-off performance, in cases where a very large flex allowance is available allowing minimal take-off thrust.  Setting climb thrust that is a little higher than the flex take-off setting has little negative impact on engine life because you are now high enough that the erosion impact is negligible so there's little engine life benefit to a slower climb to altitude and you might as well optimize climb thrust to get there faster.

It sounds weird. Given the distance criteria can be met with the climb setting, why not use it from the beginning?

I didn't think to include the biggest factor in flex usage which is physical erosion of the engine.  I added some bits that may help clarify things more.

Direct experience.  I used to be on the program.  When a Libyan and and Iraqi operator started running 900s about 10 years ago or so they were complaining pretty bitterly about how fast their engines were wearing out.  Their capts generally declined to use flex and their airlines didn't require them to.  GE and BA tech support had to convince them they had to use it if they want their engines to last anything like they expected in the fine sand environment.

In the old days (B737-200) we had to compute the thrust setting for take-off and climb manually from tables. The company policy was to set the bug for the take-off thrust position to the highest of flex/climb thrust setting if climb thrust was higher than flex reduced thrust(no auto-thrust and fadec, the PM had to manually fine-tune the thrust levers positions after the PF had roughly set the initial thrust with the throttle at the start of take-off). Because of this we indeed sometimes used climb thrust for take-off to avoid a change in thrust setting at the thrust reduction altitude. Nowadays all is done by a computer and very often you will notice that climb thrust is higher than flex thrust. It doesn’t matter anymore as the fadec does its protection job and the thrust is set by the fadec and not by the pilot. The pilot only moves the throttle position from flec to climb thus not a big deal in workload during a critical phase of flight.

Flex allows there to be various settings for Takeoff power. Climb thrust is less than Takeoff power (Takeoff is usually time-limited whereas Climb is not), but there can be several settings for Climb thrust to ensure that today's Climb is less than today's Takeoff.
Boeings use Climb, Climb1 (-10%) and Climb2 (-20%) and the FMC picks the suitable Climb power to go with today's Takeoff power. So a full derate will end up with Climb2, whereas a partial derate may need allow Climb1.

Does it mean that the lowest achievable thrust for reduced takeoff thrust is climb2 on Boeing's aircraft?

@ManuH No, you can reduce takeoff thrust by up to 25% from the rated thrust (legal maximum) by combining a thrust derating with a high assumed temperature.

@Bianfable thus take off thrust (max -25%) is lower than climb thrust (climb2, max -20%). Am I correct?

@ManuH I am sure that the lowest reduced TO thrust (-25% of rated thrust) is lower than the full climb thrust because I have actually experienced a thrust increase in a 737 when going to climb thrust. I am however not sure if Climb2 (-20% of full climb, not of rated thrust) would be even lower. I couldn't find anything in the manuals, so I didn't answer.

@Bianfable your last comment do answer my original question. Now I fail to see the rationale of using climb thrust greater than reduced take off thrust. Why not using climb thrust from the beginning?


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What is the maximal altitude at which an emergency slide can be deployed?
If I understand correctly, the mechanism to inflate an emergency slide relies on a tank of inert gas and on an "aspirator" that sucks ambient air. This aspirator works thanks to depression causes by the stored inert gas velocity (Venturi effect) while it inflate the slide.
This implies that the slide requires enough outside air to inflate correctly. I'm confident such mechanism are certified to work at high altitude international airport (I'm thinking of Lima but there may be other international airport at high altitude), but an aircraft can still crash in high altitude mountains, higher than any airport.
I wonder what is the minimal outside pressure required to inflate an emergency slide. Maybe air density is more relevant than pressure. Given the standard atmosphere model, both can be converted in altitude (either using pressure-altitude or density-altitude).
What is the maximal altitude at which the mechanism to inflate an emergency slide can work as expected (i.e. suck enough outside air to inflate the slide)?

Maybe you should consier replacing "altitude" with "elevation" in the title, since altitude implies the aircraft is flying

@expeditedescent I think altitude is correct in this instance, as altitude is in reference to the static air pressure (which changes), whereas elevation is in reference to the ground height above mean sea level. The question is in relation to the surrounding air pressure.

While it may be technically correct, I still think it is confusing.

@expeditedescent I don't know which term is better. I'm really interested in air quantity. If density is important, then I'm interested in density-altitude; if pressure is important, then I'm interested in pressure-altitude. As each of those terms use "altitude", I though "altitude" is better. But as English in not my mother tong, I surely fail to understand which one is better and why.

In the US, TSO-C69c details the FAA's technical requirements and testing protocols for evacuation slides. I can't find anything in it that talks about a maximum altitude or minimum air pressure. The closest thing I can see is this requirement in 4.2 to function correctly in a given temperature range:

The device, including its inflation system, must be demonstrated to be
capable of fully functioning when subjected to temperatures from -40
to +160 degrees F. If the device is intended for installation outside
the pressurized cabin, the device must be capable of functioning after
being stowed at -65 degrees F. The function of the device must be
demonstrated in accordance with the hot and cold soak test procedures
described in paragraph
5.9 of this appendix.

Sections 4.11 on inflation operation and 4.15 on the inflation system don't mention anything about ambient atmospheric pressure.
And the testing protocols in section 5 of the document only mention temperature, not pressure. For example, section 5.9.3 is part of the cold soak test protocol:

Deploy the device into ambient temperature conditions (typically
defined as between 65 and 85 degrees F) from the appropriate airplane
door or a suitable airplane door mock-up or module, within 10 minutes
after removal from the conditioning chamber.

4.15.2 has the only reference in the document to an aspirator system and it only requires a filter to avoid ingesting small objects.
So, unless I've missed something in the TSO - which I might have! - maximum altitude/elevation or minimum ambient atmospheric pressure aren't part of the required certification, at least in the US.

The design (as described by the OP) is inherently self-compensating, as all good systems should be when circumstances permit.
At higher altitude, which implies lower DA (let's just say density) of the outside air, the venturi-aspirator device will pull a lesser mass of air into the slide as it operates. But also,the outside air pressure is lower than at sea level hence the slide requires less of the air-gas mixture to fill completely and reach an adequate level of over-pressure to remain firmly inflated.
That aside, I am sure that back in the days when regulators were doing their jobs properly and checking engineering work, they would have picked up altitude variations as an issue while conducting HAZOPs and worked through the issues. That will be why altitude does not show up as an operational limitation.

You know, the design is the one described by wikipedia (I may misunderstand how it works). If I understand correctly, you're basically saying that the quantity of air suck by such device decreases at the same speed as (or slower than) outside pressure and thus compensate.

Yes Manu H, that is the intended meaning of my response. By the way, I support the use of the word altitude in your original question, as opposed to elevation. What we are specifically interested in here is Pressure Altitude, insofar as it influences performance of the inflation system. Elevation is irrelevant - it merely references the "parking place" of the unfortunate aircraft that is undergoing emergency evacuation. When discussing aviation we should use correct aviation terminology - as you have.


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What is the historical reasons to start Belgians registrations with "OO-"?
For many countries, the registration number begins with a obvious letter (F for France, D for German (Deutschland I guess), C for Canada,...). For other, historical reasons have already been covered here (e.g. Where did the US 'N' registration letter come from?).
But for Belgium, I find it weird to begin with OO. Is there an historical reason for that? if so, what is it?

D is for Djermany. Just say it aloud and suddenly it makes sense. ;-)

And G for UK-registered aircraft (for some reason), EI for Ireland (Eire), RA for Russia, JA for Japan, I for Italy, UR for the Ukraine, CU for Cuba, YU for Serbia (formerly Yugoslavia), LX for Luxembourg, Z for Zimbabwe...

Since nearly all aircraft have radios, the simplest solution was to make aircraft tail numbers the same as their radio callsigns. Belgium was allocated radio callsigns with the prefixes ON through OT by the ITU, though for whatever reason, they use only prefix OO for aircraft.

Well, now the question becomes: why did they get such radio callsigns?

and what is this "whatever reason"?

@Zeus Good question, but I suspect not topical here since it's not specific to aviation.


text:
What are flight envelope limits of the Stearman Model 75?
Recent monopropeller aerobatic aircraft such as the Zivko Edge 540 have impressive flight envelops limits (up to ±12G, $V_{NE}$ (230 kn) up to 4 times its stall speed ($V_s = 51kn$), low stall speed, ...). I would like to know how it compares with an old biplane with an open cockpit commonly used in air shows such as the Boeing-Stearman Model 75.
What are the flight envelops limits of the Stearman 75? I'm particularly interested in the following numbers:

load factor limits (both negative and positive G)
Speeds limits (both $V_{NE}$ (never exceeded speed) and $V_s$ (stall speed))

Not 12 G, that's for sure.

@quietflyer neither I expect the Vne to be as high as for the Zivko Edge 540.

I remember you have to throttle down when diving to collect speed for a loop in order not to overspeed the engine. RPM limit is at 1700.

Would the Pitts Special be a better pick for a "common airshow biplane"? It's a somewhat newer design than the Stearman, and more or less purpose-built to be a modest acro plane (vs. the Stearman, which was built to be a primary trainer first and foremost, but could also handle light aerobatic duty)

@UnrecognizedFallingObject I haven't heard about this aircraft before reading your comment, thus I think no it is not that common in airshow. Moreover, I'm interested by biplane with open cockpit such as the Stearman.


text:
Why is the F-35B's elevator not in neutral position when in vertical flight mode?
I was watching videos of F-35B taking off and landing vertically on ships when I realized that when transitioning from horizontal to vertical flight mode (I don't know the exact vocabulary to describe that), the horizontal elevator moves to "stick pushed full forward" position. The elevator is also put in that unusual position when transitioning from taxiing to a vertical takeoff. I know that the elevator position is not significant in vertical flight as attitude is controlled via non-aerodynamics means. Thus I wonder why this control surface is then put in that position for that flight phase instead of staying in its previous position or in a more neutral position.
I'm even more confused as this behaviors doesn't seems to be reproduced on other VTOL fighters such as the AV-8B or the Yak-141.

I'm not sure precisely why it does this, but, I agree with you that it's not because of pitching moment - see my comment to Camille Goudeseune's answer. Fig 1 in this paper might help - http://ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/finalworks2016/papers/03.2_3_366paper.pdf. It gives you the airflow pattern around a hovering F-35.

In this video you can see it better I think:

I drew on a screenshot of the video from the side where you can see the air moving under the aircraft.
Compare this to the Fig. 1 in the paper linked by "Anonymous Person" here in the comment section of your question: http://ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/finalworks2016/papers/03.2_3_366paper.pdf
As you can see angling the tailerons down significantly widens the gap between the leading edge of the tailerons and the trailing edge of the wings which allows air to be pulled along with the exhaust gases (drawn in blue). The paper calls this "entrainment".

Again I drew on another screenshot of the video and illustrated possible air currents that can freely move around the airframe: aft of the exhaust duct and to the left and right of the vertical stabilizers. As you can see the flaps are also extended which helps guide the air around the wings. There probably is a similar airflow around the front section of the fuselage where the wings are not blocking the flow and the fan is pushing the air down.
In a vertical descent or climb the tailerons also create less aerodynamic drag in this configuration than in a leveled attitude so the pitch inducing effects due to the large surfaces at the rear are reduced.
It could also be beneficial to angle the tailerons trailing edge down for a vertical landing or a short takeoff to increase the lift at the rear end by blocking the escape path for air trapped under the rear end of the aircraft. Essentially preventing the cushion of air to escape at least when in ground effect.


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How do 3d panel methods model wake-surface interaction?
I would like to know more about the general procedure used in modeling wakes for 3d panel methods.
Specifically I am looking at the effect of surfaces(fuselage and horizontal stabilizer) on the wake.
Is it better to merge wake panels with body panels or to allow the wake to clip through the body? Are there any advantages of using one over the other?

Out of curiosity -- what system are you modeling?  I have one approach that I can talk about, but it's tailored to a certain problem.

Wing-fuselage, quad panels w/ constant source and doublet if I understand correctly. The plan is to write my own code for the entire aircraft+wakes. I'd like to hear your approach regardless.

This has a couple parts to it, and it illustrates well the flexibility of well-designed panel methods.
First: panel methods can quite easily model multiple, disconnected surfaces simultaneously.  Select a proper implementation of the Kutta condition such that a trailed wake can be created, and you're all set.  The trick is to just make sure that all panels are considered as interacting with each other.  I've read a couple papers with section detailing this, including Sizing Tool for Quadrotor Biplane Tailsitter UAS by Strom.  Whereas, for one wing, you can compute the influence of all singularities on a given panel, for multiple bodies/wakes, you just compute the influence of all singularities -- regardless of what body they belong to -- on a given panel, be it on-body or in the wake.  For lifting panels (i.e., panels associated with a body that should have some nonzero trailed vorticity), the Kutta condition can be computed in the same way (e.g., for an equal-pressure, nonlinear Kutta condition, the strength of a shed wake panel is determined numerically to correspond with the shed wake of any other lifting bodies such that the Kutta condition is satisfied for all shed wake panels simultaneously).
Now that you have a wake and it is being convected in some manner, we get to the question of what to do when multiple bodies are in close proximity and the wake is predicted to convect through a body.  I've seen this treated in a rotorcraft paper (where the rotor wake is impinging on the fuselage) by Govinderajan in Contributions Toward Understanding the Effects of Rotor and Airframe Configurations on Brownout Dust Clouds.
The rotorcraft problem uses a proscribed displacement to keep the impinging rotor wake out of the fuselage.  As shown in the image below (from Govinderajan), the lines represent a straight-line vortex filament connecting various control points along the predicted wake structure.  The dashed blue line/blue dot are the predicted location for the convected wake.  Since the control point is located in the interior of the fuselage, it is moved radially outward to a point 1.1 radii from the center of the fuselage (green dot above fuselage).

A similar procedure is also followed by Lorber and Egolf in An Unsteady Helicopter Rotor Fuselage Interaction Analysis and appears to be also present in Quackenbush et. al's Vortex Methods for the Computational Analysis of Rotor/Body Interaction, all of which present methods that can predict the performance of various aircraft quite well in comparison to experimental data.  Unfortunately, I don't have a comparison to other methods for you, but this seems to be fairly common.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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How big is the radar cross section of a modern glider?
I heard somewhere that modern glider made of plastic and composite materials are less detectable than other aircraft by primary radar. I wonder how much.
What is the radar cross section of a typical modern glider such as the Schempp-Hirth Duo Discus? How does it compare to a typical GA aircraft such as a Cessna C172 and to a typical airliner such as an jet airliner such as an Embraer E190 and to a typical turboprop such as a De Havilland Canada Dash 8?

Depends on the material for the primary structure: In glass fiber gliders the radar return is mostly from things like metal control rods while in carbon fiber gliders the conductivity of carbon fiber makes them much more visible.

@PeterKämpf If the question is restricted to the duo discus, is there still multiple possible materials? (otherwise I'll restrict it another glider)

@PeterKämpf don't hesitate to tell me if there is another glider for which it is easier to answer and I'll modify the question accordingly.

When flying glass fiber gliders in the ADIZ back in the Eighties, we thought that we are invisible for radar - until some of us were invited by the military and they could tell exactly where we had been. They said a glider is about as visible as a large bird, but faster, so it can readily be identified on radar.

The aluminum instrument panel most gliders have provides a big enough target. Even if the panel is carbon, the metal in the instruments and radios does it.

@JohnK whereas the goal is not to be stealth, a glider seems to be less detectable than many other aircraft.

Obviously.  The DH Mosquito had the radar signature, to the Germans, of a small light aircraft since the only reflective components were the engines, armour plate and other bits.

Smaller than that of a normal aircraft, but very far away from a VLO/stealth one.
The RCS of various light aircraft has been measured and published here: https://apps.dtic.mil/dtic/tr/fulltext/u2/a029262.pdf
The RCS of the human body is on the order of -20 to -5 dBsm: https://ieeexplore.ieee.org/document/7027834
The glider can't hide the human inside. Extrapolating light aircraft RCS down to a glider, my best guess for the lowest-RCS gliders would be on the order of -10 to -5 dBsm, and no less than -12 dBsm or 0.05 m² due to the pilot.
The reason is that a glider still contains a lot of non-radar-transparent parts, which will reflect radar frequencies and produce a sizable RCS.
It doesn't take much to produce a radar reflection. -40 dBsm corresponds to a sphere just 1.4 cm in diameter. Anything that contains metal or water will reflect radar. Even small metal or flesh objects will produce a significant return.
To get seriously low RCS, such as -20 to -30 dBsm, below a bare human, where "real stealth" is considered to begin, you need to specifically work for it, intentionally reflecting the waves in the wrong direction, not avoiding reflections.
In particular, you need to ensure the reflection is directed into specific lobes. This isn't best achieved by radar transparency - in fact, both the F-22 and even non-VLO fighters like the F-16 intentionally coat their canopies with gold to make them radar-reflective, so as to avoid random reflections from the objects inside.

is your "guess" valid for a duo discus?

@ManuH It would be for something like the Duo Discus - for really light gliders with a minimum of metal.

and how does it compares to typical airliners, typical GA and typical turboprop?

@ManuH Way way lower. You can expect 10-20 m^2 from GA and 100-1000 m^2 from airliners. Turboprops are easily 2x the RCS of a corresponding turbofan.


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What is the purpose of classifying fighters?
Fighters are classified by generation, as explained in this answer. Given this Wikipedia sum up, some fighters are not clearly in a defined generation, especially for 4th generation subclasses. Thus people may not agree on a list of fighters that belongs to a generation.
Some of the previous generation (generation 4, 4+, 4.5, 4++) are still actively produced (e.g. Rafale, F/A-18E/F)  while the F-22 (definitely 5th generation) is no longer in production.
Airframe designed a long time ago (e.g. the F-15, F/A-18) still receive updates that make it competitive on international market (the section about potential operator of the F/A-18 on Wikipedia give an overview for the F/A-18), even in front of 5th generation fighters (at least in Canada for the F/A-18 and the F-35)
Another capability we cannot rely on to define generation is stealth. The A-12 and more precisely its variant YF-12 was stealth supersonic aircraft with weapons (AIM-47) inside internal bays, but was not a 5th generation fighter.
Moreover, since ECM can be fitted on a fighter, this fighter expose some stealth feature.
Given those facts, it seems that since the 4th generation, knowing a fighter is classified as a specific generation does not provide information about its production year, its capabilities nor its competitivity in front of other nation's fighters.
In short, since the 4th generation, generation does not provide reliable information about capabilities, features nor designed or production year.
What information does the generation of a fighter give? How useful it is to know a fighter belongs to a specific generation?

Features that the fighter planes have like stealth technology, weapon system, maneuverability, avionics technology, etc

It does provide a lot of information about its capabilities.

Aircraft recognition tells you that if you're looking at the silhouette of an A-10, you're SoL.

People like to classify things, to divide them into categories.  Categorizing allows people to get an initial broad understanding of a complicated subject more quickly.  It is therefore very useful for purposes such as marketing and articles aimed for people who aren't necessarily subject matter experts.  However, classification is necessarily a crude tool that often isn't exact, leading to confusion and arguments about where the category boundaries lie.
People who are subject matter experts, such as designers of a new fighter plane, or military analysts trying to determine the capabilities of a new secret aircraft produced by other countries, presumably don't spend much time wondering into which category to put a given aircraft.  Instead they focus their time on the details of the subject, the same details that are too much information for the person trying to get the quick grasp of the complicated subject.
To answer your questions specifically, knowing the generation of a fighter gives someone a rough idea of its capabilities, and allows the person to quickly compare a given fighter aircraft to other fighter aircraft.  Knowing the generation of a fighter is useful for people who aren't experts on the subject who need a quick broad understanding of the capabilities of historic and modern fighter aircraft, and who need to know roughly where a given aircraft fits into the large picture.

Fine, you beat me to it. Especially your last paragraph. Harumph! ;)

To be clear, there is no “generation” classification for fighters.  This was a marketing gimmick cooked up by defense OEMs to move hardware and keep Congress from axing pet projects.  The generation thing appears in the 1990s in Lockheed Martin literature about the F-22, which at the time was in danger of being stillborne.  Nobody referred to the ATF program, which sired the F-22, as a fifth generation fighter.  It was a fighter program, albeit an advanced one.
Stealth, derived from the old Anglo Saxon word “stealeth” meaning to sneak up upon and overtake by surprise, is a generic term for a wide variety of technologies called Low Observable Technologies.  They are intended to reduce the signature of a person, vehicle or structure, by identifying all means by which they can be detected by sensors.  This includes visual, infrared, radar, thermal, acoustic, etc.  LOT technology then attempts minimize the footprint made.  Stealth appears to have been a media thing, similar to how Vespa mandarinia was dubbed ‘Murder Hornet’ - it sounds scary, which gets likes, clicks, airtime and column inches.  The origins of the term stealth go back to the 1980s and were most closely attached to leaks about the Have Blue and Senior Trend programs which developed the F-117 attack aircraft.  Stealth is also often associated with radar near invisibility and refer to aircraft whose shaping was derived from Pyotr Ufimsev’s research into accurately calculating RCS size.  The B-2 “Stealth” bomber program was originally know to the public as the Advanced Technology Bomber (ATB).  The term also got traction in Tom Clancy’s book Red Storm Rising, which featured the deployment of a fictional “F-19 Ghostrider stealth fighter” on a future European battlefield against the Soviets.
In short fighters are fighters, stealthy or not.

"To be clear, there is no “generation” classification for fighters. This was a marketing gimmick cooked up by defense OEMs to move hardware and keep Congress from axing pet projects." – This is a contradiction. If this was a marketing gimmick cooked up by defense OEMs, then there is a generation classification for fighters, namely the one cooked up by defense OEMs as a marketing gimmick. Classifications are created by someone based on properties defined by someone because they are useful for someone. Someone else may create different classifications based on different properties for a …

… different use case. This classification was created by OEMs based on marketing properties in order to dazzle Congress. You and I may disagree with the use case, the chosen properties, and the classification, but that doesn't mean it doesn't exist. It's just not a classification that is useful to us, but it clearly is useful to someone, namely defense contractors lobbying Congress.

No there is not.  No engineer or principle scientist who develops these things views them by “generation”.  PR people do.  These are not technical terms.  They’re a dog and pony show to sell things, which explains why these classifications are a terrible what to group these machines in.

@CarloFelicione You're contradicting yourself again.  You say there is no such thing as generations of fighters, and then say that PR people classify fighters into generations.  Then say that these classifications (which you earlier said don't exist) are terrible. How can they be terrible if they don't exist?  "Not used by engineers" and "a terrible tool" are not the same thing as "does not exist".

In other words, people who sell things and don’t know anything about the technology of fighters make these kind stuff of distinctions to sell things.  That’s not a contradiction, that’s pointing out that fighter ‘generations’ is a nonsense term, similar to bragging about how a ‘16-bit’ video game console made it superior to its competitors and in a separate league.  Again no one familiar with the development of such things would really care,

"There's no such thing as ghosts" is not contradicted by someone pointing out that of course there's such a thing as ghosts - they're a convention of fiction and superstition.  "There's no such thing as generations of fighters" is a perfectly straightforward idiom for saying that those generations are imaginary.

@CarloFelicione that wasn't the feeling when I worked for BAE SYSTEMS in the early 2000s, some technologies we worked on definitely were considered as being too radical to put into production until the previous generation of engineers had retired. Not quite 'technology generation' but it still is a generational process.

As you can see from the Wikipedia chart, the fact that the definitions vary depending on the publication means it's all just a kind of a construct in the minds of the ones who created it.  In the end it's just a historical mental exercise with no particular applicability other than to be a useful shorthand when making a historical description of design over the years.  As far as a practical application goes, not much.

Categorizing into groups such as this helps military pilots bundle tactics together to be briefed and practiced in training.  Fighters within a generation have similar capabilities, and it is useful and practical to group them in such a way so as to be able to speak about them in broad terms.


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How precise must a pilot be when flying a holding pattern?
When ATC asks a pilot to fly a holding pattern, he expects the airplane to stay at the required altitude in the sector of the holding pattern.
For the vertical direction, the question is straightforward: how much can the aircraft deviate from its assigned altitude?
For the horizontal direction, the question is more tricky: can the pilot fly a larger than standard holding pattern (lower turn rate, straight legs twice as long as expected)? What happens if on the contrary the pilot flies the pattern in less than 2 minutes?
Bonus question: what prevents the pilot flying an 8-shape instead of an oval one if he stays inside the holding pattern area?
EDIT: my question is really about the limits. It is obvious that a deviation of few millimeter or less than 10 seconds is OK, but at what point does it start to be not allowed (in other word, what is the precision required)?
To better understand the problem I want to address, my comprehension is that as long as you stay in the assigned sector, separation is ensured without any ATC intervention. Thus, I see no added value to be as precise as possible as long as you stay in the assigned sector. You may circle closer to the holding point, the separation will still be assured. But what happens if you fly farther away but still in the assigned sector? And what happens if you fly a non standard shape but still inside the assigned the assigned sector?

Since most holding patterns/procedures are designed to align the inbound leg with a specific direction (the Intermediate Leg of an IAP for example) so that the plane is set up to continue on a desired course after the hold, what purpose would flying a figure eight achieve?

A related question is what happens when there are several planes circling, and they are incapable of maintaining the same speed.  Do slower planes use a tighter circle while maintaining their relative position behind the plane ahead?

@RayButterworth The sky is three-dimensional. Vertical separation is always used in holding patterns. It is not possible to have two aircraft in the same holding at the same altitude.

@RayButterworth - a good example of what J. Hougaard is talking about is "The Stack" in Arizona over Stanfield VOR. The same holding pattern is used by each airplane. Just at different, assigned altitudes. Also check out AIM 5–4–10 Timed Approaches from a Holding Fix for another example.

To better understand your question, could you explain what you fear would happen if a pilot crosses this hypothetical precision threshold, i.e. flies the holding too imprecisely?

@J.Hougaard precision added. If you want to edit your answer, take your time, I won't accept an answer in the next few days.

Maybe your confusion comes from the fact that a holding pattern is usually depicted on charts as a fixed oval shape. But that's really just a symbol for it. Instead, think of a holding pattern as a set of instructions. Pass the fix, turn 180 degrees, fly for x minutes, turn around again, fly to the fix. The pilots must follow these instructions, not some line on a map. The actual shape of the track flown depends on various factors such as aircraft type, speed and wind

related: What are the dimensions of the protected space provided by a holding pattern?

Flying holds does not have to be rocket science and the holds do not have to be perfect ovals.  In fact when applying proper wind correction the pattern will not be a perfect oval.  There are two main parameters you need to meet to be safe and keep ATC happy when flying a holding pattern:

Maintain your altitude:  Sometimes an ATC controller will assign multiple aircraft to a published hold but assign different altitudes.  Any deviation of altitude could reduce separation to unsafe levels.
Stay on the cleared side of the hold as much as possible:  While there is some leeway if you cross over to the uncleared side (parallel entries typically do cross over to the uncleared side) you want to stay on the cleared side as much as possible.  This not only helps with separation of traffic and terrain but also helps keep you from drifting away from the fix especially if you are encountering winds aloft.

As far as your question concerning the time it takes to fly a holding pattern that is not that important as long as you are not drifting away from the fix.  Lets say you have a tailwind when flying towards the holding fix which will turn into a headwind when turning outbound.  The goal you want to obtain is 1 minute inbound legs.  But to maintain that you will need to increase you time on the outbound leg since you are flying into a headwind and losing ground speed.  That means your holding pattern will take longer to complete but ATC will not care about that.  ATC wants you to keep you altitude and not drift away from the fix.  Whatever you have to do to safely make that happen is fine with them.
Question about 8 shaped oval:  I assume you are talking about if you accidentally turn into the uncleared side of the holding pattern and fly a figure 8 to get back on course and how to avoid that.  The rule of thumb is apply two or three times the amount of wind correction on the inbound course to the outbound course.  The reason this is done to account for the difference in turning radius when turning into the wind versus turning away from the wind.  The resulting pattern will not be a perfect oval but will look like the pattern below.  (Image borrowed from this math stack exchange post).

If you would like more info on what airspace ATC actually expects you to use for a hold click here and see wbeard52's answer.

For the 8 shape, I really meant "fly any pattern that os not an ovale but still stay inside the published ovale".

@Manu H:  It would be hard to maintain the hold pattern if the holding shape is not an oval or a wind corrected oval.  If you zigzag all over the place you are likely to drift away from the holding fix of cross over to the non-cleared side of the holding pattern.  If you want to know how much ATC is watching you in the hold, probably not too much as they would likely be busy.  However, you always want to fly the hold as accurately as possible.

Concerning the altitude, what deviation is acceptable? I imagine this is not 1mm (absurdly precise) nor 200ft (too much risk of collision) but somewhere in between, but where?

Holding procedures, like all procedures, are designed with safety margins to ensure terrain clearance and separation from other traffic/procedures. That does not mean that a pilot is free to deviate from the published procedure within those margins.

For the vertical direction, the question is straight forward: how much can the aircraft deviate for its assigned altitude?

It can't. The pilot must continuously strive to maintain the exact assigned level. If you find yourself 100ft lower than cleared, you can't just keep flying at the level - you need to take action to climb back.

For the horizontal direction, the question is more tricky: can the pilot fly a larger than standard holding pattern (lower turn rate, straight legs twice as long as expected)? What happens if on the contrary the pilot fly its pattern in less than 2minutes?

No, he can't. He wouldn't be following the published procedure.

Bonus question: What do prevent the pilot to fly 8-shape instead of oval one if he stay inside this excepted ovale?

The fact that that's not how a holding pattern is defined, and pilots generally like to follow the rules, since not doing so can have catastrophic (and not always immediately obvious) consequences.

I'm sure a procedure designer could get into the actual margins (vertical and horizontal) used when designing a holding procedure. But this question seems to be about pilot procedures, not procedure design.

I think it would be worth making the point that if a pilot does significantly deviate from the expected procedure, ATC will report him/her for pilot deviation.

@Fiddlesticks IMO that doesn't really have anything to do with the question.

@Fiddlesticks hoe much is "significant"?

Your answer seems to implicate that you can't fly the pattern in 3min59s instead of 4minutes, and you cannot deviate more than 1 mm from the published pattern. I'm sure this is not the case

@ManuH I'm not sure what you mean. Obviously it wouldn't make a difference if your legs are 1 second shorter than intended, but that doesn't mean you should aim for it. Pilots shouldn't intentially fly imprecisely, they should try to fly the procedure exactly as it is. It would make no sense to publish values for acceptable deviations, since pilots have no way to accurately determine if they are within those margins anyway. Safety margins were considered by procedure designers when they created the holding pattern, but should not be considered by pilots, who should fly it exactly as published.

It's a bit like asking how far off the centreline of the runway a pilot is allowed to land. As long as you make a safe landing, it doesn't really matter, but that doesn't mean you should aim for anything else than right in the middle.

@J.Hougaard this is exactly my question: for the runway, as long as you land on the runway, its OK, but the horlding pattern there is no such physical hard barrier

Down voted for comment saying that a holding circuit must be flown in a certain amount of time.  The goal is for 1 minute inbound legs but the pilot will have to adjust the outbound time to make that happen.

@DLH The word "time" is not mentioned anywhere in my answer (except where I quote the question!) Not sure what you mean.